Conventional steering systems for motor vehicles are sufficiently well known from practice. Nowadays, with few exceptions, practically all motor vehicles are equipped with a so-called axle pivot or Ackerman steering.
In recent times, driver assistance systems have been increasingly employed in motor vehicles in order to automate the driving task. As an example, an adaptive cruise control, a traffic congestion assistant, an autobahn or highway pilot, or a fully autonomously operating motor vehicle are mentioned. The higher the degree of automation of the driving task, the greater are the requirements placed on the reliability of the sensors, control instruments, actuators, and mechanical components involved in the tasks. The failure probabilities of all involved components must be as low as possible, for which reason often only redundant systems represent a solution. Accordingly, for steering systems, at least partially redundant steerings must be developed for highly automated driving, for example.
The effort involved in implementing the required failsafe performance for a steering system is very extensive and usually requires the doubling of processors, plugs, windings, cablings, and mechanical components. The assembly thereby becomes not only very complex, but also very expensive. In part for reasons of function and packing space, a doubling, in particular a doubling of mechanical components, is not possible at all.
The utilization of the motor vehicle brake for stabilization of a motor vehicle or for a redundant steering task is known. In this case, it is intended essentially to produce a yaw torque by braking on one side and thereby to assist the steering of the motor vehicle. In the process, individual wheels or all wheels on one side of the motor vehicle are braked.
DE 196 05 553 C1 relates to a steering system with an emergency steering system that is capable of actuating wheel brakes at different sides of the motor vehicle depending on the actuation of a steering control.
DE 10 2014 200 608 A1 relates to a method for operating a motor vehicle that has at least two wheel axles, at least one of which has steerable wheels, and at least one device for influencing a torque of the wheels individually for each wheel, wherein a steering of the wheels is assisted by a power steering depending on a desired steering angle. It is provided that the functional ability of the power steering is monitored and that, when a malfunction of the power steering is detected, a torque is influenced by the device at only one of the steerable wheels for adjustment of the desired steering angle.
It is known that the steering effect due to an influencing of the torque is greater, the more positive or the greater is the so-called steering roll radius or scrub radius of the steerable axles of the motor vehicle. A steering roll radius is understood to mean the distance from the point of intersection of an imaginary, extended line of the steering axle or steering axis through the plane of the roadway to the point of intersection of the centerline of the wheel contact surface through the plane of the roadway. If the point of intersection of the imaginary line of the steering axle with the roadway lies closer to the center of the motor vehicle than does the point of intersection of the centerline of the wheel contact surface with the roadway, then one refers to a positive steering roll radius. If, in contrast, the point of intersection of the imaginary line of the steering axle lies further away from the center of the motor vehicle than does the point of intersection of the centerline of the wheel contact surface with the roadway, then the steering roll radius is negative. A steering roll radius equal to zero is present when the two points of intersection coincide. During a braking operation with different frictional grip on the individual sides of the track, a motor vehicle with a positive steering roll radius tends to pull toward the side with better grip. A motor vehicle of this kind is highly sensitive to disruptions in the region of the steering system or of the wheels and can even break away when, for example, it travels over unevenness in the roadway. In contrast, a motor vehicle with a negative steering roll radius exhibits a self-stabilizing straight drivability. The motor vehicle can be prevented or at least impeded from braking away by a negative steering roll radius; the steering system acts in a track-stabilizing manner. Consequently, for reasons of comfort, the steering roll radius in motor vehicles is often designed to be negative to slightly positive. However, this then leads to the fact that, through braking on one side, it is also not possible to produce greater steering torques or yaw torques. Accordingly, a redundant steering system constructed in this way can contribute in only a very limited manner to influencing the direction of travel.